This invention relates to a process for isomerization of hydrochlorofluorocarbons, particularly to a process for isomerization of hydrochlorofluorocarbons to more stable isomers using fluorinated alumina catalyst.
Among the chlorofluoroalkanes (which are commonly named "flon"), perchlorofluorocarbons (containing only fluorine and chlorine, and no hydrogen) (hereinafter abbreviated as "CFC") have been widely used as refrigerant carriers, solvents, foaming agents and so on, as they are stable and safe substances.
But recently, as CFCs were found to be one of the substances to be the cause of the destruction of the ozone layer and global warming, an international agreement was concluded to restrict the production of CFCs by 2,000 A.D. completely.
Recently, research and study of substances for the substitution of these CFCs has been conducted. As the most possible substitution, hydrogen-containing chlorofluoroalkanes, namely hydrochlorofluorocarbons (hereinafter abbreviated as "HCFCs"), such as: 1,1-dichloro-2,2,2-trifluoroethane (hereinafter abbreviated as "HCFC-123"); 1-chloro-1,2,2,2-tetrafluoroethane (hereinafter abbreviated as "HCFC-124"); and 2-chloro-2,2-difluoroethane (hereinafter abbreviated as "HCFC-142b"), can be listed.
However, HCFCs have many kinds of isomers. For example, dichlorotrifluoroethane has isomers of 1,2-dichloro-1,2,2-trifluoroethane (hereinafter abbreviated as "HCFC-123a"), 2,2-dichloro-1,1,2-trifluoroethane (hereinafter abbreviated as "HCFC-123b") and the like, besides the above-mentioned HCFC-123.
In the synthesis of HCFC-123, the by-products of these isomers may sometimes be produced. Similarly, in the synthesis of HCFC-142b, 2-chloro-1,2-difluoroethane (hereinafter abbreviated as "HCFC-142a") and 1-chloro 2,2-difluoroethane (hereinafter abbreviated as "HCFC-142") may be sometimes produced as by-products.
HCFCs such as HCFC-123a, HCFC-123b and 1-chloro-1,1,2,2-tetrafluoroethane (hereinafter abbreviated as "HCFC-124a") are more unstable compounds than the HCFCs which have a high concentration of fluorine atoms around a carbon atom in the molecule such as HCFC-123 and HCFC-124, particularly those HCFCs having trifluoromethyl radicals (CF.sub.3). Stable compounds are generally preferred for most applications. For example, as in the case of use as the foaming agent of urethane foam, mixing such highly reactive HCFCs with other materials threatens to influence the quality of the product, so it is not preferable. For such application, the use of HCFCs which have a high concentration of fluorine atoms around a carbon atom, is desirable. For example, in an HCFC having more than 3 fluorine atoms in the molecule, an HCFC having the trifluoromethyl radical in the molecule (such as HCFC-123 and HCFC-124) is the most preferable. And also by way of example in dichlorodifluoroethane and chlorodifluoroethane having only 2 fluorine atoms in the molecule, 1,2-dichloro-2,2-difluoroethane (hereinafter abbreviated as "HCFC-132b") and HCFC-142b which have the difluorochloromethyl radical and have a higher concentration of halogen atoms around a carbon atom in the molecule are more preferable.
It would be quite useful if a desired HCFC could be purified and separated from its isomer mixture. However, the boiling point of such isomers are usually so close to each other that purification by the distillation process (which is industrially the most generally used separation means) is not feasible or if feasible is not economical.